Thermally cured solvent-based and water-based coatings have been widely available. A disadvantage of solvent-based coatings is a high VOC (volatile organic compound) content compared to water-based coatings. Additionally, both solvent-based and water-based thermal cure coatings, as compared to ultraviolet (“UV”) light curable coatings, can have slow rates of cure, due, in at least some cases, to a slow rate of reaction for the crosslinkers that are used. UV-curable systems often provide a much faster reaction because a UV light with certain frequencies is used to form radicals from a photoinitiator that crosslinks the vinyl groups on the resin. The UV light curing reaction can take place within a few seconds as opposed to a thermal curing reaction that can take significantly longer, including several minutes or more.
UV-curable, water-based coatings can be used in some cases. Often, in addition to radiation curable materials, these coatings often include a water dispersible polymer, such as an acid functional acrylic or polyester, which is neutralized in water with an amine. Because little or no heat is required to cure a UV-curable water-based coating, they can be applied to heat sensitive substrates, such as cellulosics and plastics.
One disadvantage that has been associated with UV-curable, water-based coating compositions is that they often exhibit poor pH and viscosity stability under the alkaline conditions that are often desirable or necessary. Often, under these conditions, the radiation curable materials, such as multi-functional (meth)acrylates, that are present hydrolyze in the presence of water, which leads to an undesired increase in the viscosity of the composition over time.
As a result, it would be desirable to provide waterborne, radiation-curable coating compositions that are viscosity stable under alkaline conditions.